This invention relates to a method of providing received signal samples to a Tacan bearing determining apparatus of the type shown and described in U.S. Pat. No. 3,836,864 entitled "Method and Apparatus for Determining Tacan Bearing" by the same inventor.
The use of Tacan permits an aircraft to determine its position relative to a ground beacon. Tacan employs a radio aerial navigation system of the polar coordinate type wherein there is a bearing facility that provides on the aircraft a meter indication of its direction in degrees of bearing from the ground beacon selected by the pilot. There is also a distance facility that provides a meter indication in nautical miles of its distance from said ground beacon. The pilot can fix his position on a chart by knowing his bearing and distance from a specific geographic point. The concepts and operational characteristics of Tacan systems are well known and a detailed description may be found in MIL-Standard 291B.
As stated in the above cited U.S. patent, it was necessary that the received antenna modulation components be sampled at a sufficiently high rate in order to reconstruct the modulation envelope. Since some 2700 pulses per second are produced by a Tacan ground transmitter and are supplied to the airborne antenna, it is necessary that the receiver be dedicated to receiving a sample for approximately 370 .mu.s for each sample. Some of the 2700 pulses per second may be distance measurement reply pulses, while others are random filler pulses, commonly referred to as squitter. While this represents a substantial decrease in duty cycle requirement over a constant duty cycle type of operation, it is still desirable to reduce receiver utilization for Tacan functions even further in order to accommodate other receiver functions; e.g. IFF. A complete description of Tacan bearing and distance measuring characteristics may be found in Electrical Communication, Technical Journal of the International Telephone and Telegraph Corporation and Associate Companies, Volume 33, No. 1, March 1956.
An apparatus which is operational at a reduced receiver sampling rate in order to free the receiver to perform additional desired functions is disclosed in the above cited patent.
Receiver output samples are applied to a group decoder which separates reference groups and applies them to a phase locked oscillator which is triggered at its zero point by the pulse groups. The oscillator output is then applied to a phase shifter which shifts the applied signal in a direction of coincidence with the envelope of the received signal samples. The outputs of the oscillator and the phase shifter are then applied to the bearing determination circuitry.
While the arrangement of the above cited patent operates to provide bearing information based on a reduced receiver sampling rate, no attention was given to the means by which these samples are derived.